1. Field of the Invention
The present invention relates to inhibitor compositions useful for preventing corrosion of ferrous metal surfaces by alkanolamine solvents used in treating sour gas streams and more particularly relates to such inhibitor compositions which contain vanadium and cobalt.
2. Prior Art
It has been a long standing commercial practice to use aqueous alkanolamine solutions (e.g. a monoethanolamine solution) to absorb acidic acids such as CO.sub.2, H.sub.2 S, COS and HCN to condition naturally occurring and synthetic gases. These treated gases may include feed synthesis gases, natural gas and flue gas. Frequently, the conditioning process is practiced by passing a 5 percent to 30 percent alkanolamine solution countercurrent to a gas stream in an absorption column to remove the acid gases. The absorbed acid gases may be later forced out of the conditioning solution at higher temperatures and the alkanolamine solution recycled for more absorbing.
Aqueous alkanolamine solutions are not themselves very corrosive toward ferrous metal equipment, however, they become highly corrosive when acid gases are dissolved therein, particularly when the solution is hot. It has been found that both general and local corrosive attack can occur. This is a particular problem in reboilers and heat exchangers where the steel is exposed to a hot, protonated alkanolamine solution. A heat transferring metal surface appears to be especially vulnerable. Previous investigation by others have revealed that under conditions corrosive products such as aminoacetic, glycolic, oxalic and formic acids were formed. The monoethanolamine salts of these acids present the possibility of increased attack upon ferrous metals.
One of the most economical and efficient methods of treating this corrosion problem is by including small quantities of corrosion inhibitors. Various metal compounds have been used by others alone or in combination with coinhibitors, for example, such as compounds of arsenic, antimony and vanadium. These metal compounds seem to be much more effective against CO.sub.2 -promoted corrosion than they are when H.sub.2 S has been absorbed in the conditioning solution.
A number of U.S. patents have been granted relating to the use of corrosion inhibitor additives. For example, the use of antimony was described in U.S. Pat. No. 2,715,605. A number of amine compounds were found to be useful in preventing corrosion by addition to petroliferous oil well fluids containing carbon dioxide or hydrogen sulfide brines, as disclosed in U.S. Pat. Nos. 3,038,856, 3,269,999 and 3,280,097. U.S. Pat. No. 3,808,140 relates to a combination inhibitor system using vanadium and antimony. Nitrosubstituted aromatic acids and acid salts, stannous salts, organo-tin compounds, benzotriazole, vanadium and antimony were used in various combinations as inhibitor systems for conditioning solutions as described in U.S. Pat. Nos. 3,896,044 and 3,959,170. The use of vanadium compounds as corrosion inhibitors for aqueous amine gas sweetening reagents is well known; for example, see H. Ratchen and C. Kozarev, Proceedings of the International Congress on Metallic Corrosion, 5th, 1972 and E. Williams and H. P. Lackie, Material Protection, July 1968, p. 21.
Pyridinium salts were found to be useful corrosion inhibitors when used together with lower alkylpolyamines, thioamides, thiocyanates, sulfides and cobalt as noted in U.S. Pat. Nos. 4,100,099; 4,100,100 and 4,102,804; as well as U.S. Pat. Nos. 4,096,085 and 4,143,119. Still another U.S. Pat. No. 2,826,516, uses soluble silicates as effective corrosion inhibitors. However, many of these corrosion inhibitor systems have not found industry acceptance because of factors such as cost and toxicity.
Other cases related to monoethanolamine gas scrubbing operations are U.S. Pat. No. 4,184,855 which uses inter-coolers and flash heat exchangers to increase the energy efficiency of the method and U.S. Pat. No. 4,183,903 which describes using cyclic ureas as anti-foaming agents in the alkaline absorption solution.
It is an object of this invention to provide an aqueous alkanolamine conditioning solution inhibitor system using components which are nontoxic relative to some of the prior art systems and which permit relatively higher amine concentrations and thus higher carbon dioxide loading making for a more efficient process.